(a). Field of the Invention
The present invention relates to a bonding pad structure, more particularly, to a bonding pad structure for improving impedance matching.
(b). Description of the Prior Arts
Along with the retirement of the outdated electronic products, the operating frequency of the newer generation electronic products are gradually become higher and higher so as to fulfill the demands of the consumers. For instance, the operating frequency of a package structure corresponding to a radio-frequency (RF) circuit or high-speed circuit commonly will be employed in GHz range.
Thus, the package structure corresponding to the electronic product having high operating frequency is required to have better electrical characteristics so as to stabilize the high-frequency electronic product while functioning. Please refer to FIG. 1, which is a cross-sectional view depicting a package structure of a RF circuit. In the package structure 100 as seen in FIG. 1, the signal transmitted between the chip 110 and the substrate 127 is accomplished using the longer metal wire 125 to couple the chip 110 with a lead located on the substrate 127. On the other hand, part of the grounding of the signal between the chip 110 and the substrate 127 is accomplished using the shorter metal wire 120 to couple the chip 110 directly with the chip-carrying die pad 130 located on the substrate 127 so that a shorter signal loop between the chip 110 and the substrate 127 can be achieved for enabling the package structure 100 to have better electrical characteristics and heat-flux capability.
However, in the aforesaid high-frequency package structure 100, the total area between the chip 110 and the substrate 127 defined using the loop corresponding to the longer metal wire 125 and the shorter metal wire 120 is still too large that the electrical characteristics of the package structure 100 will show high inductance and high impedance, moreover, while the package structure 100 is operating in high frequency, a sever signal reflection will occur because of the impedance mismatching between the forgoing loop, also the return loss of the loop is decreased.
For improving the electrical characteristics of the high-frequency package structure 100, that is, enabling the loop between the chip 110 and the substrate 127 to have both matching inductance and matching capacitance, or to have a better-matched return loss and insertion loss, the prior art normally will install a NMOS transistor or a PMOS transistor inside the chip 110 of the package structure 100 as a voltage-stabilizing capacitor. Nevertheless, the installation of the NMOS transistor or the PMOS transistor had confined the signal bandwidth and the accuracy of the package structure to haven many other applications.
In addition, for further bettering the electrical characteristics of the high-frequency package structure 100, the Metal-Insulator-Metal (MIM) process is used for installing the aforesaid NMOS transistor or PMOS transistor inside the chip 110 of the package structure 100. However, the cost of the MIM process is too high to be commonly implemented.
The present invention is a bonding pad structure for improving impedance matching that can optimize the impedance matching of the loop between the chip and the substrate inside the package so as to improve the electrical characteristics of the package structure by increasing the capacitance of the loop through a simple structural refinement of the bonding pad structure.